Digital phase modulation, or phase shift keying (PSK), has found significant use in terrestrial and satellite communication systems, particularly at microwave frequencies. Digital phase modulation has the advantage that, for a given carrier to noise ratio, the bit error rate of a digital phase modulated system is smaller than that of pulse amplitude modulation or frequency shift keying systems. Common forms of phase shift keying systems use 2, 4, 8 or 16 phase states; e.g., a bi-phase system would have phase states at 0.degree. and 180.degree., a quadrature system would have four phase states each separated by 90.degree., and so forth. Modulators capable of generating such phase shift states may generally be grouped into series, parallel, and commutating categories, depending upon the manner in which the phase shift is developed. The present invention may be considered to fall within the class of parallel modulators.
In a parallel modulator, the outputs of two amplitude controllable bi-phase modulators may be combined in quadrature to form the basis for the output signal; by control of the amplitude and phase--either 0.degree. or 180.degree.--of each of the bi-phase modulators, an output signal having an arbitrary phase angle can be produced. It is possible for a parallel modulator to be structured to provide amplitude modulation as well as phase shift keying, and to allow continuous phase modulation.
At microwave frequencies, the construction of workable modulators becomes difficult because of the phase errors introduced by parasitic reactances; as a result of these reactances, the amplitude signal is difficult to control without substantially varying the phase from the desired 0.degree. or 180.degree.. Because attenuation is obtained in typical diode ring mixers by turning the diodes off, the diode junction by itself presents a particularly significant capacitive reactance at microwave frequencies.